The present invention relates to the structure of a hand display type electronic watch equipped with a so-called coin-shaped lithium battery cell or like flat cell which has a relatively large energy capacity per unit volume and a large diameter compared with its thickness (meaning a relatively large area as viewed in a plan).
A current trend in the art of watches of hand display type is the use of a quartz oscillation circuit as a time standard oscillator and a stepping motor or like electromechanical transducer for driving a wheel train and associated hands.
Conventional hand display type quartz watches, however, require cell replacement which is repeated in every short period of time. This is because they are powered by so-called button-shaped silver battery cells (silver oxide cells or silver peroxide cells) having an energy capacity per unit volume smaller than that of the coin-shaped lithium cells; the service life of a button-shaped silver cell is generally as short as two to three years. Another problem attributable to the use of such a silver cell is that its inherent structure is liable to cause leakage of the electrolyte which would invite serious failures in the circuitry, wheel train and other sections of the watch.
Meanwhile, coin-shaped lithium cells have recently been spotlighted as power sources for various portable electronic instruments including electronic watches of digital display type and electronic desk-top calculators. Compared with a button-shaped silver cell, a coin-shaped lithium cell has a smaller rate of natural deterioration caused by self-discharge, a large energy capacity per unit volume, and a far less possibility of leakage of the electrolyte. Despite these advantages, a lithium cell has not so far been installed in electronic watches of hand display type.
A lithium cell increases its internal resistance as its thickness is increased. With this in view, a lithium cell particularly for use in a watch must be provided with a coin-like shape which is relatively thin and has a sufficiently large diameter compared with the thickness.
A coin-shaped lithium cell preferable for a watch may be about 16-25 mm in diameter, about 0.6-2.5 mm in thickness and 0.8-4.0 g in weight. In this connection, a watch for men employs a plate whose diameter generally ranges from about 23 mm to 28 mm. Considering these dimensions, it will be understood that the practical use of a coin-shaped lithium cell with a hand display electronic watch is difficult unless how the cell should be arranged and supported in the watch is settled.
Different from a digital display electronic watch, a hand display electronic watch includes a wheel train driven by an electromechanical transducer and located in a position adjacent to a substantially central part of the plate in a plan view and intervening between the plate and wheel train support in a sectional view. It will thus be apparent from the relationship between the diameter of the plate and that of the coin-shaped lithium cell that the lithium cell in a hand display electronic watch must be layed over the space which has accommodated the wheel train and wheel train support. To meet this requirement, there must be settled two different problems altogether: how the various elements of such a watch should be layed out to prevent a disproportionate increase in the overall thickness of the watch and intricacy of its construction, and how the cell should be supported to cope with impact forces applied from the outside to the watch without imparting the resultant impacts from the cell directly to the wheel train which constitutes a movable part of the watch.
Generally, a watch is accepted as more valuable as its thickness decreases. In case where use is made of a coin-shaped lithium cell having the aforementioned dimensions, the ratio of the diameter d of the cell to that D of the plate is approximately within the range of 0.65.ltoreq.d/D .ltoreq.0.95 so that the cell in the watch ineviatably overlaps a major part of the other elements of the watch. This requires a specially designed arrangement and structure which avoids an increase in the thickness of the watch.
A hand display watch also differs from a digital display watch in that it includes a part of the electromechanical transducer, wheel train or like movable section which performs rotation or like mechanical motion through a delicately designed support structure. It follows that, where it is intended to overlay a heavy lithium cell on such movable sections, there must be avoided a structure which permits an impact force attributable to the presence of the cell to directly reach and damage the movable sections. Indeed, it has already been proposed to provide an unusual structure to the back cover and hold a flat cell inside the back cover, or to interpose a second back cover between the movement and a first back cover and accommodate a flat cell between the first and second back covers. Though these known structures succeed in preventing an impact load from the cell from reaching the movable sections when an impact due to dropping or the like is applied to the watch, they add to the intricacy of construction and therefore to the production cost while making the entire watch very thick. For these reasons, hand display electronic watches using lithium cells have not been put to practical use.